TT Bike

How much do you gain with a TT bike over a conventional road bike? Assuming a good aero position on a road bike nets you a sustained 22 ~ 23 mph over a given course, what would a reasonably well set up TT bike with aero helmet gain in terms of mph?

id imagine the difference between a properly fitted road bike for TT'ing using aerobars and an aero helmet would be small. i think much of the advantage is in aero positioning using aerobars regardless of bike type.

How much do you gain with a TT bike over a conventional road bike? Assuming a good aero position on a road bike nets you a sustained 22 ~ 23 mph over a given course, what would a reasonably well set up TT bike with aero helmet gain in terms of mph?